1. Field of the Invention
The present invention relates to a liquid-sealed type semiconductor pressure sensor and a method for manufacturing such a pressure sensor in which a pressure transmitting medium such as insulating oil fills a pressure detecting chamber and sealed by a sealing diaphragm.
2. Description of the Related Art
Although not prior art to the present invention, Japanese Patent Application No. Hei 6-6269 laid open as publication No. 7-20115 on Aug. 11, 1995, discloses the liquid-sealed type semiconductor pressure sensor illustrated in FIG. 7, in which a semiconductor pressure sensing element 1 is disposed in a recess of a connector housing 3, a main housing 4 having a sealing diaphragm 5 previously secured thereto is placed on the connector housing 3, and, thereafter, filler oil 7 as a pressure transmitting medium is filled through a filling orifice 3b into a pressure detecting chamber. The orifice 3b is then sealed by a sealing material such as a spherical rubber member (e.g. rubber ball), an annular rubber packing member 16 having annular projections on its peripheral surface, or the like. Thus, the filling orifice 3b is sealed up air-tightly.
However, the filling orifice represents a relatively complicated structure for the purposes of manufacture and installation, and increases the size of the pressure sensor. An increase in the number of sealing portions also may cause decrease in reliability. Moreover, such features necessitate additional members and work for sealing, thereby resulting in an increase of the device's manufacturing cost.
The temperature characteristics of the relationship of the liquid-sealed pressure sensor will be discussed next with reference to FIG. 6.
When no excessive internal pressure exists in the pressure detecting chamber, the characteristic curve of the internal pressure to the temperature is shown by curve D in which the pressure in the pressure detecting chamber at an ambient temperature (25.degree. C.) is indicated by point A (pressure in the pressure detecting chamber is 0), and the curve has a temperature coefficient corresponding to thermal expansion of the filler oil 7 and other elements. The curve is almost linear over the temperature range of practical use (-30 through 125.degree. C.), and may be compensated to conform to a curve D' by a control circuit which converts the change in the resistance of the pressure sensitive element into voltage and compensates the variation in the pressure sensor signal due to temperature change (compensation 1') by a corresponding change in the voltage. As a result, a pressure sensor which operates on the curve D' or which is free of temperature variation may be provided. However, if the volume of the filler oil 7 contained in the pressure detecting chamber is too much for the sealing diaphragm 5 and causes the diaphragm 5 to bend and separate from the pressure sensitive element 1, the pressure in the pressure detecting chamber changes on a curve C and comes to a point B (pressure P in the pressure detecting chamber is a positive value) at the ambient temperature (25.degree. C.). The curve C rises sharply at a portion in the higher temperature range; in other words, the pressure P in the pressure detecting chamber rises sharply. That is, the pressure in the pressure detecting chamber P changes linearly as temperature changes in a lower temperature range, however the pressure in the pressure detecting chamber increases more sharply than the temperature, and the proportional relation therebetween fails at the higher temperature range. As a result, the aforementioned temperature compensation by the control circuit (compensation 1 for the curve C') is not effectively applicable, nor is shifting of the output voltage of the control circuit (compensation indicated by an arrow 2) effective.
When bubbles exist in the pressure detecting chamber, positive or negative pressure applied to the sealing diaphragm 5 is not accurately transmitted to the pressure sensitive element due to the bubbles which contract or expand under the pressure applied thereto, thereby causing a pressure loss. Thus, in order to transmit the pressure applied to the sealing diaphragm 5 to the pressure sensitive element 1 accurately, an appropriate volume of the filler oil 7 without air bubbles is required for proper functioning of the pressure transmitting medium.